wezterm: allow using {} (an empty table) to represent None in lua

`wezterm.action{ExtendSelectionToMouseCursor=nil}` doesn't produce an
`ExtendSelectionToMouseCursor(None)` value because a table value of
`nil` is equivalent to that key not being present and lua sees just
an empty table.

Instead we need to accept `ExtendSelectionToMouseCursor={}` a valid
way to indicate an `Option::None` which is what this commit does.

Due to weirdness that I haven't had a chance to run down, passing
that value through `wezterm.action` doesn't produce the intended
value, so I'm adjusting the docs to show to specify an alternative
syntax for this as part of this commit.

refs: https://github.com/wez/wezterm/issues/282

docs/config/keys.markdown

@@ -158,7 +158,7 @@ that order.
 | Triple Left Down | `NONE`   | `SelectTextAtMouseCursor="Line"`  |
 | Double Left Down | `NONE`   | `SelectTextAtMouseCursor="Word"`  |
 | Single Left Down | `NONE`   | `SelectTextAtMouseCursor="Cell"`  |
-| Single Left Down | `SHIFT`   | `ExtendSelectionToMouseCursor=nil`  |
+| Single Left Down | `SHIFT`   | `ExtendSelectionToMouseCursor={}`  |
 | Single Left Up | `NONE`   | `CompleteSelectionOrOpenLinkAtMouseCursor`  |
 | Double Left Up | `NONE`   | `CompleteSelection`  |
 | Triple Left Up | `NONE`   | `CompleteSelection`  |
@@ -735,6 +735,25 @@ Extends the current text selection to the current mouse cursor position.
 The mode argument can be one of `Cell`, `Word` or `Line` to control
 the scope of the selection.
 
+It is also possible to leave the mode unspecified like this:
+
+```lua
+return {
+  mouse_bindings = {
+    {
+      event={Up={streak=1, button="Left"}},
+      mods="SHIFT",
+      -- Note that there is no `wezterm.action` here
+      action=ExtendSelectionToMouseCursor={},
+    },
+  }
+}
+```
+
+when unspecified, wezterm will use a default mode which at the time
+of writing is `Cell`, but in a future release may be context sensitive
+based on recent actions.
+
 ## OpenLinkAtMouseCursor
 
 If the current mouse cursor position is over a cell that contains

src/scripting/serde_lua/mod.rs

@@ -403,8 +403,16 @@ impl<'de, 'lua> Deserializer<'de> for ValueWrapper<'lua> {
     where
         V: Visitor<'de>,
     {
-        match self.0 {
+        match &self.0 {
             Value::Nil => v.visit_none(),
+            Value::Table(t) => {
+                let mut iter = t.clone().pairs::<String, Value>();
+                if iter.next().is_none() {
+                    v.visit_none()
+                } else {
+                    v.visit_some(self)
+                }
+            }
             _ => v.visit_some(self),
         }
     }
@@ -962,4 +970,26 @@ mod test {
             unknown variant `Invalid`, expected `Foo` or `Bar`",
         );
     }
+
+    #[test]
+    fn test_option_mode() {
+        #[derive(Serialize, Deserialize, Clone, Debug, Eq, PartialEq)]
+        enum MyEnum {
+            Foo,
+            Bar,
+        };
+        #[derive(Serialize, Deserialize, Clone, Debug, Eq, PartialEq)]
+        enum AnotherEnum {
+            ThisOne(Option<MyEnum>),
+        };
+
+        let lua = Lua::new();
+        let res: AnotherEnum =
+            from_lua_value(lua.load("{ThisOne=\"Foo\"}").eval().unwrap()).unwrap();
+        assert_eq!(res, AnotherEnum::ThisOne(Some(MyEnum::Foo)));
+        round_trip(res);
+
+        let res: AnotherEnum = from_lua_value(lua.load("{ThisOne={}}").eval().unwrap()).unwrap();
+        assert_eq!(res, AnotherEnum::ThisOne(None));
+    }
 }